In recent years, techniques of using an automated tape bonding process in packaging integrated circuit chips have been developed. The technique is used to replace other lead frame wire bonding methods used in packaging integrated circuit chips.
In a tape bonding process, an integrated circuit chip is directly bonded to a foil-type lead frame that is usually less than 0.5 mm in thickness. The technique is also referred to as tape automated bonding or TAB. In a TAB process, a bare copper, a gold or tin plated copper or a copper/plastic laminated tape is first prepared by etching leads into it at positions corresponding to the gold plated bumps over aluminum bonding pads on an integrated circuit chip. The tape is then fed into an inner lead bonder, which is an apparatus equipped with a thermode, i.e., a heated instrument that presses the chip and the tape together. The inner end of the leads are bonded to the bumps on the integrated circuit chip by compressing them under the heated thermode in a single operation. The integrated circuit chip and bonded leads can then be excised out of the tape for connection to a circuit board.
As shown in FIG. 1, a length of copper tape 10 is loaded into an inner lead bonder machine between thermode 12 and anvil 14. An integrated circuit chip 16 is provided and positioned on the flat top 18 of anvil 14 with gold tipped or solder tipped contact bumps 20. Contact bumps 20 face upward. An electric resistance heating coil 22 is used to heat thermode 12 of the inner lead bonder. A suitable copper tape used in this process is a 0.0028 inch thick non-plated copper of 2 oz. weight supplied by the Minnesota Mining and Manufacturing Co. A gold or tin plated copper tape, or a copper/plastic laminated tape may also be used.
In the conventional tape automated bonding technique, contact bumps 20 are made by first building up an aluminum pad to about 1200 .mu.m in height. The aluminum contact bump is then covered with sputtered layers of titanium and an alloy of tungsten. Each layer is about 200-300 .mu.m thick and has a combined thickness of about 500 .mu.m. A thin layer of about 400 .mu.m of gold is then sputtered on top of the contact bump. In a final processing step, a gold layer of 0.001 inch (or 25,400 .mu.m) in thickness is plated on top of the sputtered gold layer.
Thermode 12 is then lowered to contact tape 10 and integrated circuit chip 16. FIG. 2 shows thermode 12 in a closed position wherein etched finger leads 24 in tape 10 are pressed against gold tipped or solder tipped bumps 20 between thermode surface 26 and anvil surface 18. The usual time, temperature, and bonding pressure used are 0.8 sec, 525.degree. C., and 200 gms/bump respectively. However, it is to be understood that bonding pressure can be adjusted based on the heat input from heating coil 22 and the time desired for each particular bonding process.
In a conventional TAB method using gold plated aluminum bumps, the high rigidity of the metal necessitates a high bonding force between the IC chip and the lead frame. The high bonding force frequently causes crack formations in the chip at the base of a bump.
It is therefore an object of the present invention to provide a TAB bonding method to bond a lead frame to an IC chip that does not have the drawbacks and shortcomings of the prior art methods.
It is another object of the present invention to provide a method of TAB bonding wherein a lead frame is bonded to bumps on an IC chip that have reduced rigidity.
It is a further object of the present invention to provide a method of TAB bonding a lead frame to bumps on an IC chip wherein the bumps are constructed of a composite material.
It is another further object of the present invention to provide a TAB bonding method wherein a lead frame is bonded to bumps on a IC chip that are constructed of a polymer/metal composite.
It is yet another object of the present invention to provide a TAB bonding method in which a lead frame is bonded to composite bumps on an IC chip without causing any crack formation in the chip.
It is still another object of the present invention to provide a TAB bonding method wherein a lead frame is bonded to a composite bumps on an IC chip without any structural damage occurring during the bonding process.